Changing/Lowering the FPS in Gas Blowback Rifles and/or Pistoles That Don't Have After Market Parts Available.

[ClearCut Airsoft]

Hi all I have recently been hooked by airsoft and am now obsessed with gbbrs. However, The more interesting replicas I find the more I realise I will not be able to use them due to their fps. For example, the vfc MP7 and UMP 45/9 seam really cool but both shoot around the 380-400 fps mark out of the box with no upgrades/replacement parts to change this. As much as I love H&K I would like my replicas to be more than just pretty wall hangers.

I would love to hear any and all opinions on what I could do or whether I should just stick to RA-Tech npas compatible guns such as my WE 999c (G36c) which is my first and only rifle thus far (seen below).

View attachment 75168

 

[Chev Chelios]

You can shorten the inner barrel or purchase one with a wider ID. GBB work on a flute valve system resting on a spring, gas travels through the system and when enough presser is built up, the valve is forced forward redirecting the gas to propel the slide/bolt to the rear. Just like an AEG you can cut or use a softer spring in the nozzle to reduce the muzzle energy. 

You can use HFC134A (duster gas) to drop the FPS even further but you lose recoil and thus some of the fun factor.

 

[Cyberlawyer]

Its pretty easy to adjust FPS on a GBBR even if there are no after market parts. In terms of options, I have listed some below in order of easiest to hardest.

1. Use weaker gas - no modification required just buy some HFC135A.

2. Fit a shorter/wider barrel. If you are happy to re-crown the barrel yourself (its pretty easy with a Dremel) you can just shorten the inner barrel you have a bit at a time until you get the desired FPS. A shorter inner barrel gives the BB less time to accelerate before the pressure disperses at the end of the barrel hence less FPS. A wider barrel will let more air past the sides of the BB reducing the pressure for a similar effect. Please be aware that cutting a barrel does not result in a linear drop in FPS (cutting an inch off of a 14 inch barrel will have much less reduction in FPS than cutting an inch off of a 6 inch barrel) and once barrels get below about 3 inches accuracy can suffer as the BB may not have time to stabilise in the air flow after the hop has been applied (the exact length at which these weird results will manifest depends on a number of factors such as FPS, weight of the BB, amount of hop, gas volume, but 3 inches is a good rule of thumb at 350 fps).

3. Fit a weaker valve spring. Your can buy loads of different springs on e-bay you just need the same diameter and approximate length and then its just trial and error on the spring strength/tension. A weaker spring allows the valve to close faster directing less gas down the barrel and more into the blowback mechanism. This has the double advantage of dropping the FPS and increasing the bolt cyclic rate, but a faster bolt can increase wear on the working parts. Note you can also cut the valve spring, but I don't recommend it as it is not a long spring with a lot of preload (unlike an AEG gearbox spring) and you still need enough spring tension to hold the valve fully open at the start of the cycle or cycling will become erratic so the scope for adjustment is quite limited.

4. You can made your own DIY NPAS using a small drill and a grub screw (there are several videos on YouTube). This effectively holds the valve partially closed so when gas pressure is applied in doesn't have so far to travel so closes faster (see results above).

I hope that helps.

C